The present invention relates to a hydraulic control system for an automatic transmission having a torque converter with a lock-up mechanism.
In the case of automatic transmissions having a torque converter with a lock-up mechanism, i.e., a mechanism to directly connect a pump impeller to a turbine runner, it is preferable to actuate the lock-up mechanism from a speed ratio as low as possible. Thus, in the case of an automatic transmission having three forward speed ratios and one reverse speed ratio, it is preferable to keep the lock-up mechanism actuated during operation in each of second and third forward speed ratios. However, if the transmission is shifted between the second and third speed ratios with the lock-up mechanism left actuated, substantial shocks will take place during shifting because torque variation takes place in the circumstance where an input shaft of the transmission is mechanically connected to a crankshaft of an engine. To avoid the shocks, an electronic control system has been needed which senses the commencement and completion of a shifting and generates electric signals for controlling the actuation of the lock-up mechanism. The electronic control system, however, requires a plurality of sensors and actuators, thus resulting in an enormous cost increase. If the same is to be carried out hydraulically, an extremely complicated hydraulic control system is necessary, thus failing to meet the demands for less cost and space saving.